This invention relates to high contrast silver halide materials and particularly to those of the graphic arts type.
For many years the very high contrast photographic images needed in the graphic arts and printing industries were obtained by developing a xe2x80x9clithxe2x80x9d emulsion (usually high in silver chloride content) in a hydroquinone, low sulphite, xe2x80x9clithxe2x80x9d developer by the process known as infectious development. However, such low sulphite developers are inherently unstable and are particularly inappropriate for machine processing.
More recently, emulsions containing hydrazine nucleating agents have been used and processed in a high pH (about pH 11.5) developer with conventional amounts of sulphite, hydroquinone and possible metol or a pyrazolidone. While such a process is better than the low sulphite lith process, the developer still has less sulphite than is optimal and a high pH requirement for it to function correctly. Such a solution is not as stable as is desirable. Additionally high pH solutions are environmentally undesirable because of the care needed in handling and disposing of the effluent.
EP-A-0 531014 claims high contrast materials sensitive to more than one spectral region comprising one layer of emulsion sensitive to one region and another emulsion layer sensitive to another region. Each spectral sensitivity requires its own emulsion layer. In a comparative test (page 6 lines 9 to 19) a blend of differently dye sensitised emulsions was used as the sole emulsion layer. This material showed loss of speed, especially if one of the spectral regions is in the infrared. The explanation for this appears to be that the sensitising dyes are being desorbed from their silver halide grain hosts.
EP-A-0 208514 claims high contrast materials containing a hydrazide wherein there are two distinct populations of grains differing in grain volume. In the comparative examples (outside the scope of the invention claimed) emulsion blending is described. Some of the blends are dye sensitised. However the blending takes place before the addition of any sensitising dye so that the blended emulsions described have grains which are either all dye sensitised or all not dye sensitised. Blends of sensitised and unsensitised grains are not mentioned.
A further improvement in the area of high contrast materials is the introduction of a lower pH process (below pH 11) using hydrazides active at this low pH together with the use of a contrast booster compound, for example, one of the boosters described in U.S. Pat. No. 5,316,889 or an amine booster as described in U.S. Pat. Nos. 4,269,929; 4,668,605; and 4,470,452. The hydrazides proposed for use in such materials are described for example in U.S. Pat. Nos. 4,278,748; 4,031,127; 4,030,925 and 4,323,643 and in European Patent 0,333,435.
In most photographic materials the type and size of the silver halide grain determines the speed of the material while also affecting the covering power of the silver image formed therefrom. In general smaller sized grains provide higher density and covering power than larger ones. In some materials therefore there has to be a balance struck between speed and covering power in high contrast materials another balance between vigorous development and pepper fog (which occurs if development is too vigorous) needs to be achieved.
EP-A-0 682 288 describes high contrast photographic materials containing a silver halide emulsion layer and containing in the emulsion layer or in an adjacent hydrophilic colloid layer a hydrazide nucleating agent wherein the emulsion contains silver halide grains which are spectrally sensitised and silver halide grains which are not spectrally sensitised the sensitising dye(s) being chosen so that it does (they do) not become desorbed from the spectrally sensitised grains. This is said to provide savings in sensitising dye and improvements in ease of manufacture while retaining desirable density, low Dmin, high covering power and avoidance of pepper fog.
The inventors of the present invention have found that grains with octahedral character do not nucleate efficiently in current nucleated graphic arts developers eg Kodak registered Trade Mark)RA 2000 chemistry. This implies that films containing only 111 tabular emulsions do not nucleate efficiently. On the other hand tabular emulsions would be desirable due to their inherent potential for high speed for a given grain volume. A solution to this problem has now been invented by using a dual emulsion system in which the emulsion grains of octahedral character, or more specifically 111 tabular character, are applied as a coating in conjunction with a non octahedral emulsion.
According to the present invention there is provided a negative-working high contrast pbotographic material comprising a support bearing a silver halide emulsion layer, containing in the emulsion layer or in an adjacent hydrophilic collide layer, a hydrazide nucleating agent characterized in that the emulsion layer contains silver halide grains of octahedral character which are spectrally sensitised and silver halide grains which are not octahedral in character.
The use of tabular grains as causer emulsions enhances the absorption characteristics of the dyes used so that as well as providing high photographic sensitivity, the dye peaks are broadened which is very usefull for films designed to cater for exposure devices of differing wavelengths.